每次运行循环时生成一个随机数

Question

该程序模拟掷骰子的游戏，该游戏有两个骰子。在第一个掷骰中，如果骰子的总和是7或11，则玩家获胜。如果掷骰子的总和是2、3或12，则玩家将输。其他掷骰被称为“点”，然后游戏继续。在随后的每次掷骰中，如果玩家再次掷出“点”，则获胜。玩家通过掷骰7输了。任何其他掷骰将被忽略，游戏继续进行。

我使用函数roll_dice(void)生成了1到6之间的两个随机数，然后将它们加在一起。我知道问题在于，似乎无法在while循环中生成另一个数字。

#include <stdio.h>
#include <stdbool.h>
#include <time.h>
#include <stdlib.h>
int roll_dice(void);
int main(void)
{
    int a, pointer;
    int win = 0, lose = 0;
    a = roll_dice(); /*Assigns the sum of the 2 generated random numbers to a*/
    printf("%d\n", a);
    printf("You rolled: %d\n", a);
    if (a == 2 || a == 3 || a == 12) { /*First roll's loss condition*/
        printf("You lose!");
        lose = 1;
    }
    else if (a == 7 || a == 11) { /*First roll's win condition*/
        printf("You win!\n");
        win = 1;
    }
    else {      /*Otherwise the first roll is assigned to a 'pointer' */
        pointer = a;
        printf("Your point is: %d\n", pointer);
    }
    while (!win || !lose) {   /*While the game is still on */
        a = roll_dice();      /*The problem is here, the roll_dice got called for the second time, yet still produces the same sum */
        printf("You rolled: %d\n", a);
        if (a == 7) {
            printf("You lose!\n");
            lose = 1;
            break;
        }
        if (a == pointer) {
            printf("You win!\n");
            win = 1;
            break;
        }
    }
    return 0;
}

int roll_dice(void)       /*Generates 2 random number from 1 to 6 and add them*/
{
    int a, b;
    srand((unsigned) time(NULL));
    a = rand() % 5 + 1;
    b = rand() % 5 + 1;
    return a + b;
}

由于每次都会返回两个相同的数字，因此游戏大部分时间都是赢了：（